I am adding a new VLAN to an existing trunk port between two Cisco Catalyst switches (3750's). In the process of adding the new VLAN, it appears that I've removed the existing allowed VLANs on the trunk... How is this possible?
Existing trunk port configuration:
SW-LAB-1#show run int g1/0/49 Building configuration... Current configuration : 255 bytes ! interface GigabitEthernet1/0/49 description SW-LAB-2 G1/0/48 switchport trunk encapsulation dot1q switchport trunk native vlan 10 switchport trunk allowed vlan 10,20 switchport mode trunk switchport nonegotiate ip dhcp snooping trust end
I used the following syntax to also allow VLAN 30:
SW-LAB-1#conf t Enter configuration commands, one per line. End with CNTL/Z. SW-LAB-1(config)#interface g1/0/49 SW-LAB-1(config-if)#switchport trunk allow vlan 30
However now, my running config on g1/0/49 is missing VLANs 10 and 20!
<SNIP> switchport trunk allowed vlan 30 </SNIP>
What am I missing?